Traditional mobility scooters, as typically used by the elderly, disabled or infirm, are manufactured in two formats: three-wheeled versions, and four-wheeled versions.
As illustrated in FIGS. 1a and 1b, known three-wheeled scooters 10 have two wheels 12, 13 at the rear of the vehicle, and one wheel 14 at the front. The rear wheels 12, 13 normally serve as the driven wheels, although some smaller models provide drive from the front wheel 14. The steering for the vehicle is provided by the front wheel 14, which is steerable.
Three-wheeled scooters have some advantages, and some disadvantages. Advantageously, having only one wheel at the front allows the user 18 to place their feet on the floor either side of the wheel 14, thus giving an ideal seating position (see FIG. 1a). The three-wheeled configuration also allows maximum maneuverability when used in a confined area.
However, three-wheeled scooters have the disadvantages that, if the vehicle should hit an obstacle such as a kerb, pot hole or steep camber (angle α), the rear wheel can lift, tipping the vehicle over. This situation can also arise when braking and turning at the same time. In short, three-wheeled scooters are inherently unstable. To compensate for this instability, small caster wheels 16, 17 may be fitted either side of the floor pan. The problem with such caster wheels is that they decrease ground clearance, and in some situations momentarily cause loss of steering by lifting the front wheel 14 off the ground. The normal mounting position of the caster wheels 16, 17 is approximately 150 mm back from the front wheel 14. This is not an ideal position, it is a compromise, and in many situations will not stop the vehicle from tipping over.
As illustrated in FIGS. 2a and 2b, known four-wheeled scooters 20 have two wheels 22 at the rear, and two steerable wheels 24, 25 at the front. Generally the two rear wheels are driven. The two front steerable wheels 24, 25 are linked by the steering mechanism, such that they steer in unison.
Four-wheeled scooters also have some advantages, and some disadvantages. The primary advantage is that, by having a wheel in each corner of the vehicle, it is inherently very stable.
However, disadvantageously, because both front wheels 24, 25 have to turn when steering, the floor pan area is restricted (see FIG. 2a). To minimize the area of the floor pan required by the front wheels for steering, the steering lock is reduced from that of a three-wheeled scooter, and consequently maneuverability of the four-wheeled scooter is restricted.
An additional problem is the knee position and knee angle of the user 26 on a four-wheeled scooter (see FIG. 2a). Because of the restricted floor pan area of a four-wheeled scooter, the user's knee is articulated such that the lower leg is in a substantially vertical position, which places pressure on the hip joint (as illustrated by the solid line 26 in FIG. 2a). This also has the effect of the person positioning themselves in a “sit up and beg” position, which is considered a bad posture, especially for a disabled person with arthritic joints. To compensate for this problem with the seated position, some manufacturers of four-wheeled scooters encourage the user to place their feet on the cover above the front wheels (as illustrated by the dotted line 28 in FIG. 2a) to elevate the knee joint angle, but this has been found to aggravate the hip joint angle and to place pressure on the user's back.
A further problem is that four-wheeled scooters do not have a “null zone” in which the steering centres itself. With a car, if the steering wheel is released during cornering, it will centralise itself. However, four-wheeled scooters do not have this feature, and consequently the steering is very active and can cause “grabbing” of the front wheels, whereby the steer angle of the front wheels is varied in an uncontrolled manner. This can be unnerving if the rider has low upper body strength, and in the extreme can be dangerous.
Thus, from the above discussion, it will be appreciated that a mobility vehicle should ideally have the following characteristics:                1. a tight turning circle (conventionally provided by three-wheeled scooters, but not by four-wheeled scooters)        2. stability (conventionally provided by four-wheeled scooters, but not by three-wheeled scooters)        3. good seating posture for the user (conventionally provided by three-wheeled scooters, but not by four-wheeled scooters)        4. active (self centralising) steering        5. compact dimensions        